Launching systems for unmanned aerial vehicles (UAV) are designed to create enough force and speed that the UAV can be ejected into the air. The general concept behind a UAV launching system is to take a vehicle from rest to the desired flight velocity in a minimum distance, without imparting destructive forces to the vehicle. UAV launcher systems for vehicles weighing thirty pounds or more typically use a pneumatic or pneumatic/hydraulic system as the prime propulsion system.
The traditional approach to take-off for many UAV's (including taxi, accelerate, lift-off, and climb) often requires a distance of 200 feet or more. This traditional take-off minimizes the acceleration (g-load) on the vehicle because it is accelerated over a longer distance. However, there is a desire to design systems that can obtain the desired launch velocity in less than 50 feet in some instances. For example, on shipboard applications and other instances, space may be limited. In addition, the landing gear associated with runway take-off and landing operations adds weight and thus requires more power and fuel to sustain flight operations.
However, the use of a launcher that allows shorter distance to achieve flight (because the acceleration is faster) generally means higher g-loads. There are often expensive electronics on-board the UAV that cannot withstand such high g-loads. Other limitations to launch parameters include a minimum launch velocity or a maximum space to launch. The design and optimization of the launcher then becomes a balance of launch stroke length, vehicle acceleration, vehicle weight to be launched, and launch angle.
The power source for the UAV launchers designed to date has typically been a self-contained power source in the form of a closed loop hydraulic/pneumatic system, which stores energy when dry nitrogen is compressed in an accumulator by pumping in hydraulic fluid. The hydraulic pump is usually driven by either an electric motor, a gasoline engine, or by a multi-fuel engine.
Historically, closed loop hydraulic-pneumatic systems have proven to be the most reliable and repeatable under the widest range of environmental conditions. To prevent condensation at extreme temperatures, dry nitrogen (GN2) is used, instead of air, to fill the “pneumatic” side of a piston accumulator. The nitrogen is pre-charged to a pre-determined pressure. A hydraulic pump then pressurizes the hydraulic side of the accumulator piston, which compresses the nitrogen and raises the launch pressure. Once the optimal launch pressure is reached, the system holds the pressure via check valves until launch is initiated. Upon launch initiation, the valve opens, the nitrogen expands, pushing the fluid out of the accumulator and into the cylinder. This accelerates the cylinder piston, the reeving cable, shuttle and vehicle.
However, there are some limitations and problems associated with pneumatic launchers. For example, there is typically an accumulator associated with the system that must be pre-charged to a specific pressure to achieve the desired launch velocity for a given UAV weight. If a different speed is required or if the weight of the UAV varies (due to fuel load or ordinance), the pre-charge pressure must be adjusted accordingly. This generally requires that gas (typically air or dry nitrogen) either be bled from or added to the system via a separate gas bottle. The need to vary the pressure adds to system complexity and potentially increases the overall system weight (e.g., if a gas bottle positioned on-board the launcher is used).
With a pneumatic launcher, it can be also difficult to control the g-load imparted to the UAV when the pressure is released into the mechanical drive components at the initiation of the launch cycle. These spikes in the g-load at the beginning of the launch cycle can have potentially disastrous impacts on the UAV and the on-board electronics and other systems. These initial g-load spikes can be mitigated through control valves that release the hydraulic fluid from the accumulator into the drive cylinder in a controlled fashion. However, these valves are often expensive and add weight to the overall system.
Additionally, many UAV launchers are used in an expeditionary mode, where they need to be mobile and capable of being transported to a location for deployment. In some cases, they may be mounted to the back of a truck. In other cases, they may be trailer mounted and either towed into position or slung from the underside of a helicopter and air lifted into position. In most cases, the overall size and weight of the launcher system must be minimized to ensure that it can fit within certain aircraft or transport containers. The main drive components of a hydraulic/pneumatic launcher (accumulator, pump, launch cylinder, gas bottle, reservoir, etc.) add substantial weight to the system, and weight is a primary limitation to mobility of the system.
With any hydraulic/pneumatic system, leaks are always a concern. Loss of gas pressure or a hydraulic leak could potentially shut down operations. Once fielded, it is unlikely that there will be access to gas cylinders to address leaks in the system.
Launch timing can also be an issue with a hydraulic/pneumatic system. Depending on the differential between the pre-pressure and final launch pressure, the size of the pump and amount of hydraulic fluid to be moved, it can take up to several minutes to bring the system up to launch pressure. The UAV is typically mounted on the launcher, and its engine is running during this pressurization time, making it susceptible to overheating.
Reset can be another challenge presented by a hydraulic/pneumatic system. Resetting a hydraulic/pneumatic launcher after completion of a launch requires that the shuttle be pulled back into the launch position. This may take several minutes because, as the shuttle is pulled back, the hydraulic fluid needs to be pushed out of the cylinder and back into the reservoir. The time required to reposition the shuttle negatively impacts the overall cycle time.
One launcher design that does not use a hydraulic system is described in U.S. Pat. No. 4,678,143. The launcher described by this patent uses a flywheel that provides the energy required for the launch sequence. The flywheel is spun up by a small electric motor that is powered by a generator, and an electric clutch engages the flywheel when the launch cycle is initiated. The flywheel drives a cable drum that wraps cable around the drum during the launch sequence. One of the disadvantages with this launcher is that the flywheel may take several minutes to come up to launch speed. Another disadvantage is the requirement of a generator as a power source, which can add a great deal of weight to the system.